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Abstract
This study investigates the Lagrangian acceleration and velocity of fluid particles in swirling flows via direct numerical simulation. The intermittency characteristics of acceleration and velocity of fluid particles are investigated at different swirl numbers and Reynolds numbers. The flatness factor and trajectory curvature are used to analyse the effect of Lagrangian intermittency. The joint probability density function of Lagrangian acceleration and turbulence intensity is shown to explain the augmentation effect of Lagrangian intermittency by the strongly swirling levels under the relatively low intensity of turbulence. In addition, the correlation between the Lagrangian acceleration and the turbulence intensity is enhanced as the swirl level increases. It shows the important effect of swirl on the motion behaviour of fluid particles in the strongly swirling flows.
Acknowledgements
We thank LetPub for its linguistic assistance during the preparation of this manuscript.
Nomenclature
| Scalars | = | |
| d | = | Diameter of jet inlet (m) |
| n | = | Group index (−) |
| Ni | = | Number of grid points (−) |
| Ns | = | Number of simulated time steps (−) |
| p | = | Pressure (Pa) |
| Re | = | Reynolds number (−) |
| S | = | Swirling number (−) |
| ui | = | Velocity components (m/s) |
| u | = | Velocity magnitude (m/s) |
| U0 | = | Inflow mean velocity (m/s) |
| Ua | = | Azimuthal velocity (m/s) |
| xi | = | Cartesian coordinates (m) |
| β4 | = | Flatness factor (−) |
| δ | = | Spacing resolution (m) |
| δτ | = | Time step (s) |
| η | = | Kolmogorov length scale (m) |
| λ | = | Integral length scale (m) |
| λ2 | = | A criterion for vortex core (−) |
| ν | = | Kinematic viscosity (m2/s) |
| ρ | = | Correlation coefficient (−) |
| χ | = | Trajectory curvature (/m) |
| ω | = | Vorticity (/s) |
| Δt | = | Time step (s) |
| Vectors | = | |
| a | = | Acceleration vector (m/s2) |
| u | = | Instantaneous velocity vector (m/s) |
| U | = | Mean velocity vector (m/s) |
| Operator | = | |
| < > | = | Mean/average |
| Cov | = | Covariance |
| Subscripts | = | |
| 0 | = | Sampled |
| L | = | Lagrangian |
| n | = | Normalized |
| rms | = | Root of mean square |
| // | = | Parallel |
| ⊥ | = | Vertical |
| Acronyms | = | |
| CRZ | = | Central recirculating zone |
| JC | = | Jet core |
| = | Probability density function | |
| Reg | = | Region |
| VB | = | Vortex breakdown |